i've known i had a slow air leak for a while. It takes over nite to air down and i've been all over the brake lines and tubing with a soapy bottle trying to find the leak. no luck.
today, while working on my dash board gauges, i noticed an audible leak that you don't hear with the dash board closed up.

the elbows into the pressure gauge seem to be my leaky culprits and i'll replace them in a day or so.

while inspecting the system in the dash. the parking brake control knob also was leaking out of the exhaust port. i took it out and cleaned up the valve which seemed to fix that leak.

when i started to reassemble the park valve in the dash, i noticed what looks like a broken air fitting on a line that doesnt attach anywhere.

each air line is a different color so i looked on the internet and found this:

Perhaps a dump for the park-brake. Even if there is no fitting, the valve may have been replaced. Also, could be an airline for the Stop-sign. Or possible for the wipers? or air-door actuator? I'm not sure what your bus has on it that requires air.. Or perhaps it was "plumbed" for one of these features that could have been used, but wasn't.

Dang: How loud was your air-leak? My park brake valve leaks a tiny bit, just enough to hear if it is dead-quiet. If I give it just a tiny tap, it seals off. Somewhere I leak though; Over 4 or 5 days my pressure drops down to 50 or 60 psi.....

lol, it wasn't loud. i was just surprised how quiet it was. the whoosh of the airbrakes valve vents outside now and there was some "hee haw" of an air valve that would go on for 30-60 minutes after i shut her down.

my bus has never held air overnight. its nice to find these leaks that have been hiding.